Civil Engineering Reference
In-Depth Information
sampler over the depth interval of 15 to 45 cm (6 to 18 in.). The reason the number of
blows required to drive the SPT sampler for the first 15 cm (6 in.) is not included in the
N
value is that the drilling process often disturbs the soil at the bottom of the borehole, and
the readings at 15 to 45 cm (6 to 18 in.) are believed to be more representative of the in
situ penetration resistance of the granular soil.
Factors That Could Affect the Test Results.
The measured SPT
N
value can be influ-
enced by the type of soil, such as the amount of fines and gravel-size particles in the soil.
Saturated sands that contain appreciable fine soil particles, such as silty or clayey sands,
could give abnormally high
N
values if they have a tendency to dilate or abnormally low
N
values if they have a tendency to contract during the undrained shear conditions associ-
ated with driving the SPT sampler. Gravel-size particles increase the driving resistance
(hence increased
N
value) by becoming stuck in the SPT sampler tip or barrel.
A factor that could influence the measured SPT
N
value is groundwater. It is important
to maintain a level of water in the borehole at or above the in situ groundwater level. This
is to prevent groundwater from rushing into the bottom of the borehole, which could loosen
the granular soil and result in low measured
N
values.
Besides the soil and groundwater conditions described above, many different testing
factors can influence the accuracy of the SPT readings. For example, the measured SPT
N
value could be influenced by the hammer efficiency, the rate at which the blows are
applied, the borehole diameter, and the rod lengths. The different factors that can affect the
standard penetration test results are presented in Table 5.1.
Corrections for Testing and Overburden Pressure.
Corrections can be applied to the test
results to compensate for the testing procedures (Skempton 1986):
N
60
1.67
E
m
C
b
C
r
N
(5.1)
where
N
60
standard penetration test
N
value corrected for field testing procedures
E
m
hammer efficiency (for U.S. equipment,
E
m
is 0.6 for a safety hammer and
0.45 for a doughnut hammer)
C
b
borehole diameter correction (
C
b
1.0 for boreholes of 65- to 115-mm diam-
eter, 1.05 for 150-mm diameter, and 1.15 for 200-mm diameter hole)
C
r
rod length correction (
C
r
0.75 for up to 4 m of drill rods, 0.85 for 4 to 6 m
of drill rods, 0.95 for 6 to 10 m of drill rods, and 1.00 for
10 m). Some
researchers recommend this correction be discontinued (Daniel et al. 2006).
N
measured standard penetration test
N
value
For many geotechnical earthquake engineering evaluations, such as liquefaction analy-
sis, the standard penetration test
N
60
value [Eq. (5.1)] is corrected for the vertical effective
stress
v
0
. When a correction is applied to the
N
60
value to account for the vertical effective
pressure, these values are referred to as (
N
1
)
60
values. The procedure consists of multiply-
ing the
N
60
value by a correction
C
N
in order to calculate the (
N
1
)
60
value. Figure 5.11 pre-
sents a chart that is commonly used to obtain the correction factor
C
N
.
Another option is to
use the following equation:
(
N
1
)
60
C
N
N
60
(100
v
0
)
0.5
N
60
(5.2)
where (
N
1
)
60
standard penetration test
N
value corrected for both field testing procedures
and overburden pressure
C
N
correction factor to account for overburden pressure. As indicated in
Eq. (5.2),
C
N
is approximately equal to (100
v
0
)
0.5
, where
v
0
is the vertical
